Glass has been used as front covers for electronic devices. Electronic device manufacturers are now desiring that the back covers of electronic devices are also made of glass and that the back covers meet the same high dimensional accuracy and surface quality as the front covers. It is possible to make the front and back covers separately with the requisite dimensional accuracy and surface quality and then assemble them together. However, this adds extra steps to the manufacturing process and may result in loss of dimensional control. The alternative is to manufacture a monolithic glass sleeve, where the front side of the glass sleeve will serve as the front cover and the back side of the glass sleeve will serve as the back cover. Many electronic devices incorporate flat displays. Thus the monolithic glass sleeve would need to have a cross-sectional profile that can accommodate a flat display. In general, this cross-sectional profile will have flat sides that can be arranged in parallel to the flat display. The flatness of the flat sides would also have to meet stringent requirements specified by the electronic device manufacturers.
It is known to make glass tubing and then convert the glass tubing into containers. Thus one practical approach to making a monolithic glass sleeve would be to make a glass tubing having the desired cross-sectional profile and then cut the glass tubing into glass sleeves. Methods for forming glass tubing from molten glass are known. The most common ones are the Danner process, the Vello process, and the downdraw process. These processes are described in, for example, Heinz G. Pfaender, “Schott Guide to Glass,” 2nd ed., Chapman & Hall, 1996. These processes are typically used to form glass tubing with a round cross-sectional shape. Extrusion can be used to form glass tubing with a non-round cross-sectional shape, e.g., a cross-sectional shape that could have flat sides. However, extrusion involves tool contact with the glass surface, which could diminish the surface quality of the glass.